A variety of elastomeric coverings can be used to coat rolls in applications in which the rolls require protection. Polyurethane elastomers are used as roll coatings in applications where abrasion and tear resistance, good load bearings characteristics, high hardness, and solvent resistance are required.
In steel mill applications, for example, large numbers of these rolls are used for guiding, tensioning, and otherwise engaging steel strip during rolling and pickling applications. The rolls are subjected to the strong chemical agents used for pickling and cleaning the strip. Similarly, in paper mills, polyurethane-coated metal rolls are used for supporting and conveying paper through high pressure nips of paper making machine assemblies such as press rolls, calendar stacks, and the like. The rolls used in paper mill operations require a protective cover whose thickness can be varied depending on the roll's anticipated use. Being able to control the thickness of the protective cover is, therefore, desirable.
Different methods have been developed for applying coatings to metal rolls. U.S. Pat. Nos. 3,520,747; 3,646,651; and 4,368,568 describe multistep methods for multilayer coating of rolls.
U.S. Pat. No. 4,571,798 describes a method wherein paper mill rolls are coated by spraying a quick-gelling polyurethane onto a core.
Ruprecht et al, Polyurethanes World Congress 1991 (Sep. 24-26, 1991), 478-481, describe rotational casting techniques useful for producing roll coverings using fast-reacting polyurethane elastomer systems. In these systems, the polyurethane reaction mixture is metered through a movable mixing head which travels at constant speed in the axial direction along the rotating roll core, a short distance above its surface. The polyurethane reaction mixture solidifies very quickly, in a matter of seconds, to produce a polyurethane coating with a thickness buildup of 4-5 mm. Additional layers of the polyurethane reaction mixture are applied until the desired thickness of polyurethane coating is achieved.
Rotational casting reduces the number of steps involved in roll coating. However, if the polyurethane reaction mixture gels too slowly, the polyurethane coating will drip off the roll. If the polyurethane reaction mixture is formulated to gel quicker, the polyurethane can gel in the head of the mixer or ridges can develop on the outer surface of the roll requiring machine finishing of the polyurethane coating on the surface of the roll.
Thickening agents, such as fumed silica, can be added to the polyurethane reaction mixture in a rotational casting procedure, to thicken the coating and eliminate dripping. However, the use of thickening agents alone can often result in the polyurethane reaction mixture becoming too thick to flow or in the polyurethane coating being applied in undesirable thicknesses.
It is therefore an object of the present invention to provide a new polyurethane composition useful for coating cylindrical objects. It is a further object of this invention to provide a polyurethane compostion which can be used in a rotational casting process.